Fix empty STDIN handling in user import-csv (#619)

Author: ekamran

features/user-import-csv.feature

@@ -140,7 +140,6 @@ Feature: Import users from CSV
       | Bob Jones         | contributor          |
       | Bill Jones        | administrator,author |
 
-  @broken
   Scenario: Importing users from STDIN
     Given a WP install
     And a users.csv file:

src/User_Command.php

@@ -1150,6 +1150,10 @@ public function import_csv( $args, $assoc_args ) {
 
 				$csv_data[] = $data;
 			}
+
+			if ( empty( $indexes ) && empty( $csv_data ) ) {
+				WP_CLI::error( 'Unable to read content from STDIN.' );
+			}
 		} else {
 			$csv_data = new CsvIterator( $filename );
 		}